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This article lays out a new method to measure the antioxidant capacity of some flavonoids. The methodology developed is based on the kinetics of the reaction of the antioxidant substrate with the superoxide radical (O(2)(*-)). A cyclic voltammetric technique was used to generate O(2)(*-) by reduction of molecular oxygen in aprotic media. In the same experiment the consumption of the radical was directly measured by the anodic current decay of the superoxide radical oxidation in the presence of increasing concentrations of antioxidant substrate. The method was statistically validated on flavonoid monomers and on the standard antioxidants: trolox, ascorbic acid and phloroglucinol. The linear correlations between the anodic current of O(2)(*-) and the substrate concentration allowed the determination of antioxidant index values expressed by the substrate concentration needed to consume 30% (AI(30)) and 50% (AI(50)) of O(2)(*-) in given conditions of oxygen concentration and scanning rate. The fidelity of the method was examined intraday and interlaboratories.

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The thermodynamic ionization constants (pK(a)(1), pK(a)(2), and pK(a)(3)) of ginkgolide B (9H-1,7a-(epoxymethano)-1H,6aH-cyclopenta[c]furo[2,3-b]furo-[3',2':3,4]cyclopenta[1,2-d]furan-5,9,12-(4H)-trione, 3-tert-butylhexahydro-4,7b,11-trihydroxy-8-methyl-) in aqueous solution have been settled by pH-metric and NMR studies. The three macroscopic pK(a) values as well as the water solubility and the water/n-octanol partition coefficient have been extracted from pH-metric data by means of a nonlinear regression methodology. NMR spectroscopy provided confirmation of the values of the macroscopic constants, information about the effective ionization pathways, and an estimation of the proportions of the various forms under physiologically relevant conditions.